Trunks saying "Will you train me even harder?" implies they had already been training together, though perhaps not with the intent that Trunks fight the androids. Since he was already SSJ in the manga, then he definitely would have already been training with Gohan to get to that point. Since everyone else died when Trunks was a baby, someone had to have taught him how to control his ki, how to fly, etc. In the manga chapter, the only fight we see Gohan in is when he flies off to confront the androids without Trunks and gets himself killed. Seventeen and Gohan have a short conversation during the fight that includes (in the Viz English translation):Trunks: I can't take this! I can't just let the Artificial Humans keep having their way! I want to become stronger! I want to get stronger and beat them! Gohan-san, please! Will you train me even harder?
Now I'm getting into merging the anime special and the manga chapter together, but bear with me. If we assume that the anime special took this exchange and created the fight at the amusement park where Gohan's face is scarred and he loses his arm, then we can assume that Gohan and Trunks have the conversation at Capsule Corp when Trunks is thirteen (this is backed up by the anime special indicating in the beginning after the title, "And now, thirteen years later..."). So at a minimum, Gohan trained Trunks seriously for a year. How much more time can we say they trained together than that? I don't know. Based on what Bulma says in both the manga chapter and the anime special, and how Trunks asks Gohan not to say anything after their conversation at CC, it sounds like Bulma doesn't want Trunks trying to fight the androids. So if Gohan was training Trunks for more than that one year, they must have been hiding it from Bulma and/or doing it against her wishes.Seventeen: It's been a while, Son Gohan. A whole year, hasn't it? I'm amazed you escaped all that...
Gohan: I've been training hard, you won't beat me this time. It's your turn to lose!
Based on all of that, I don't think they trained seriously much more than the one year we can assume they did if we accept the anime special as canon-compliant. I know, this is a minor thing to write up a whole thread on, but I'm trying to build a reasonable timeline for Trunks's own personal history, and how much time he spent with Gohan is part of it. What does everyone else think?